This invention relates to iron oxide pigments of greatly improved thermal stability, and enhanced color quality.
Iron oxides exhibit good resistance to chemical attack, excellent light fastness and, in addition, are nontoxic. The combination of nontoxicity, high performance, and relatively low cost means that iron oxide pigments are in increasingly high demand. The principal drawback associated with many of the iron oxide pigments is their lack of thermal stability. While the red iron oxide, hematite, is stable to over 1000.degree. C., the yellow oxide, hydrated ferric oxide, and the black oxide, ferroso-ferric oxide, are not thermally stable. Hydrated ferric oxide when heated above approximately 175.degree. C. begins to lose its water of hydration and change from the desired yellow color to red or brown. Similarly, ferroso-ferric oxide when heated to above approximately 100.degree. C. begins to oxidize and change from its desired black color to an undesirable red or brown. This thermal instability of the hydrated ferric oxide and ferroso-ferric oxide limits the applications where these very desirable pigments can be used.
There is a very substantial need for a thermally stable yellow pigment. The two principal reasons for this are:
1. The widely-used, thermally stable yellow pigments available today contain toxic heavy metals such as lead, antimony and chromium (VI).
2. The yellow pigments available today are relatively expensive; i.e., from five to ten times as expensive as conventional iron oxide pigments.
Thus, hydrated ferric oxide would constitute a very useful alternative if it possessed sufficient thermal stability.
Others have sought to overcome the problems of lack of thermal stability, and other deficiencies, in iron oxides. For example, the addition of soluble stabilizers during particle formation and growth is practiced in the production of magnetic iron oxides (Hund - U.S. Pat. No. 3,382,174; Wooditsch - U.S. Pat. No. 3,931,025). In these cases, arsenate or orthophosphate ion is incorporated into the hydrated ferric oxide crystal for the purpose of improving the magnetic properties of the subsequent Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3 formed (Hund, Wooditsch), as well as improving the particle shape, temperature resistance and production rate of .gamma.-FeOOH (Hund).
Sherman, in U.S. Pat. No. 3,946,134, asserts that a 5-50% continuous, dense, protective skin of aluminum or magnesium phosphate will improve the light, heat and chemical stability of lead chromate and several organic pigments.
Abeck, (U.S. Pat. No. 3,652,334) teaches the use of surface coating .alpha.-FeOOH with water soluble orthophosphate, metaphosphates, pyrophosphates, and oxyacids of phosphorus to reduce sintering during the high temperature reduction step of transforming .alpha.-FeOOH to magnetic .gamma.-Fe.sub.2 O.sub.3. These coatings represent water soluble phosphates adsorbed onto the surface of the particle rather than insoluble metaphosphates precipitated onto the surface of the particle and, while they may reduce sintering, they do not effect the dramatic improvement in color and crystal stability that the insoluble metaphosphate coatings of this invention do.
Impurities introduced into the hydrated iron oxide crystal, especially at levels as high as 25% (Hund), degrade the color when compared with a pure hydrated iron oxide pigment. This is also true of high level surface coatings (e.g., Sherman, 50%). The low level surface treatment of the present invention, as low as 1% insoluble metaphosphate, does not alter the crystal morphology of the hydrated ferric oxide since the particle has been completely formed before coating. The result is a pigment with excellent color quality and superior thermal stability.
The product of this invention exhibits remarkable thermal stability, up to 18% greater than conventional yellow and black iron oxides. This dramatic improvement in thermal stability means that the hydrated ferric oxides and ferroso-ferric oxides of this invention can be used in a majority of the plastic, coil coating and powder coating applications where conventional yellow and black iron oxides cannot be used.